add tests for plural forms function

src/core/prelude/i18n/helpers.ts

@@ -174,7 +174,7 @@ export function pluralizeText(
  * @param n - The number to evaluate for pluralization.
  * @param rules - Plural rules object. If undefined, a default rule set is used.
  */
-function getPluralFormName(n: number, rules?: CanUndef<Intl.PluralRules>): keyof Required<PluralTranslation> {
+export function getPluralFormName(n: number, rules?: CanUndef<Intl.PluralRules>): keyof Required<PluralTranslation> {
 	if (rules != null) {
 		return <keyof PluralTranslation>rules.select(n);
 	}

src/core/prelude/i18n/spec.ts

@@ -7,6 +7,7 @@
  */
 
 import { pluralizeText, resolveTemplate } from 'core/prelude/i18n';
+import { getPluralFormName } from 'core/prelude/i18n/helpers';
 
 describe('core/prelude/i18n', () => {
 	const rules = new Intl.PluralRules('en');
@@ -22,6 +23,22 @@ describe('core/prelude/i18n', () => {
 
 	const formNames = <Array<keyof typeof forms>>Object.keys(forms);
 
+	describe('pluralization forms detection', () => {
+		it('detecting plural form without Intl rules', () => {
+			expect(getPluralFormName(0)).toBe('zero');
+			expect(getPluralFormName(1)).toBe('one');
+			expect(getPluralFormName(2)).toBe('few');
+			expect(getPluralFormName(5)).toBe('many');
+		});
+
+		it('detecting plural form using Intl rules', () => {
+			expect(getPluralFormName(0, rules)).toBe('other');
+			expect(getPluralFormName(1, rules)).toBe('one');
+			expect(getPluralFormName(2, rules)).toBe('other');
+			expect(getPluralFormName(5, rules)).toBe('other');
+		});
+	});
+
 	describe('text pluralization', () => {
 		it('using pluralization constants to choose the right form', () => {
 			formNames.forEach((form) => {